1. Field of the Invention
The present invention relates to beam scanning systems for use in electrophotographic type image forming apparatuses, and more particularly, to a beam scanning system for diffracting and deflecting beams emitted from light sources using a diffractive deflection disc, in which the deflection disc has double-sided diffraction patterns to increase a deflection angle, and simultaneously to avoid bowing of the beams.
2. Description of the Related Art
In general, beam scanning systems are employed by electrophotographic image forming apparatuses for use in forming an electrostatic latent image on a photosensitive medium such as a photoreceptor web by, for example, scanning beams emitted from a laser scanning unit and a light source. Recently, a multi-beam scanning system which diffractingly deflects beams emitted from light sources by adopting a rotary deflection disc, instead of by adopting a rotary polygon used in a conventional beam scanning system, has been introduced. FIG. 1 shows a schematic configuration thereof.
Referring to FIG. 1, the beam scanning system includes a light source 10 and a deflection disc 11 rotatably mounted over the light source 10. The deflection disc 11 rotates rapidly by being coupled to a driving motor 12, and a plurality of sectors having hologram patterns are formed on the surface thereof.
A beam emitted from the light source 10 is diffracted by the hologram patterns while passing through a rotating deflection disc 11. Referring to FIG. 2, because hologram patterns 11a are formed to have different diffraction angles according to the rotation angle of the deflection disc 11, beams that are emitted from the same light source 10, are diffracted at different angles with the rotation of the deflection disc 11, to create a single scanline of the beams. The beams diffracted by the diffraction disc 11 are deflected by a plurality of reflecting mirrors 13 and 14, so that its traveling direction is changed.
The reflected beams come to pass through a beam correction means. In general, a condensing mirror 15 for condensing and reflecting the beam, and a hologram optical element 16 for diffractingly transmitting the beam to direct the beam toward a photosensitive medium (not shown) such as a photoreceptor web. Alternatively, the beam correction means may be replaced with an F-.theta. lens (not shown) that corrects the focal position and scanwidth of the beam. The F-.theta. lens corrects aberrations of the beam scanned in a primary scan direction and sets the form of the beam as the deflection disc 11 rotates.
Through the above operations, beams emitted from the light source 10 can form a scanline on the photoreceptor web in the primary scan direction, that is, in a direction perpendicular to the traveling direction of the photoreceptor web.
In the beam scanning system having the above configuration, there are many difficulties in manufacturing the deflection disc 11. That is, due to a rapid rotation of the deflection disc 11 for operation, its operation becomes less stable with an increase in the size of the deflection disc 11. However, because the deflection disc 11 needs such high-density hologram patterns 11a to diffract beams, there is a limit to reducing the size of the deflection disc 11. As the size of the deflection disc 11 becomes smaller, a more precise processing technology is required to form the hologram pattern 11a thereon.
Another problem of the conventional beam scanning system adopting the deflection disc mentioned above is bowing of the scanline, where a straight scanline cannot be created because beams are diffracted by the rotation of the deflection disc 11 while the light source is fixed. Thus, the conventional beam scanning system needs an external optical device, that is, a bowing correction reflecting mirror, to correcting bowing. If a bowing generating source does not arise from the deflection disc or if the bowing can be corrected by the deflection disc itself, the number of parts for a scanning system can be reduced.